1. Field of the Invention
Embodiments of the invention relate to seismic exploration in marine environments.
2. Description of the Related Art
Seismic exploration operations generally utilize a seismic energy source to generate an acoustic signal that propagates into the earth. The acoustic signal is partially reflected by subsurface seismic reflectors in the earth, which may include interfaces between subsurface lithologic or fluid layers that may be characterized by different elastic properties. The reflected signals are detected and recorded by seismic receiver units located at or near the surface of the earth, thereby generating a seismic survey of the subsurface. The recorded signals, or seismic energy data, can then be processed to yield information relating to the lithologic subsurface formations, identifying such features, as, for example, lithologic subsurface formation boundaries.
Generally, the method for detection and recording of seismic signals is similar on land and in marine environments; however, marine environments present unique challenges due to the body of water overlaying the earth's surface. Seismic exploration operations in marine environments are typically conducted from the deck of one or more seismic exploration vessels, such as floating platforms or ships. The seismic exploration vessels typically provide storage and transportation for a plurality of seismic receiver units and associated operational equipment. Seismic exploration in deep water typically uses seismic sensor units deployed from the deck of the seismic exploration vessel to be placed on or near the seabed. The seismic sensor units are typically coupled to a rope or cable that is placed in the water and allowed to fall through a water column to the seabed. These seismic sensor units are part of systems typically referred to as Ocean Bottom Cabling (OBC) or Ocean Bottom Seismometer (OBS) systems, wherein data from a seismic survey may be received.
When performing a seismic survey in marine environments, a specific area of the seabed is intended to be explored. Generally, a plurality of seismic sensor units are coupled to a cable and deployed from a deployment vessel to form an array or grid of seismic sensor units on the seabed. Typically, the accuracy of the seismic survey depends upon controlled placement of the sensor units on the seabed. The placement of the seismic sensor units deployed in this manner may be affected by many factors, some of which include position of the deployment vessel in the water, wind speed, speed of the deployment vessel, and underwater currents caused by naturally occurring current flows and/or turbulence generated by the deployment vessel, among other factors.
Conventional deployment methods typically utilize variations in the speed of the deployment vessel to control the deployment of the cable, which can lead to inconsistent deployment of the cable and inconsistent placement of seismic sensor units. For example, if the deployment speed of the vessel is not controlled accurately or responsively, the cable deployment may be erratic, which may cause seismic sensor unit placement inconsistencies. As an example, slack may build up in the cable between the vessel and one or more seismic sensor units that have not fallen to the seabed, which may make the towed cable susceptible to drift by currents. Another example includes slack build-up in the cable between seismic sensor units. Yet another example includes dragging of the seismic sensor units along the seabed. All of these examples can lead to unintended drift or movement of the seismic sensor units, possibly placing them outside of the intended areas to be tested.
Thus, there exists a need for an improved method and apparatus for deploying seismic sensor units to be placed on a seabed from a seismic exploration vessel.